It is desirable to prevent transient overvoltages or longer duration overvoltages on the secondary winding of a current transformer to prevent damage to the transformer and to avoid high voltages on the apparatus to which the secondary winding is connected as such voltages could be hazardous to personnel. A transient overvoltage might result, for example, from a temporary disturbance in the power system, and an overvoltage of longer duration might result, for example, from an open circuit in the secondary winding of the current transformer due perhaps to maintenance or due to a simple mechanical break. While overvoltages should be prevented to avoid damage to the transformer or to avoid high voltages which might be hazardous to personnel, any protective apparatus connected to the transformer should not affect the accuracy of the current transformer.
It is known to use a spark gap device connected across the secondary winding of a current transformer to limit the voltage on the secondary. One advantage of this device is that the gap can be selected so that the arc is initiated only when the voltage is above the working range of voltages and consequently there is no current drawn over the working range so that the accuracy is not affected. In addition, the device is re-useable. Once the overvoltage disappears, the arc stops. On the other hand, while the spark gap devices handle transient voltages, they frequently are unable to properly limit overvoltages of longer duration resulting, for example, from an open secondary circuit. In addition, the arcing causes wear of the terminals, increases the spark gap, which proportionally increases the let-through voltage.
It is also known to connect non-linear resistors across the secondary winding of a current transformer to limit overvoltages. The non-linear resistors draw a leakage current which affects the accuracy of the transformer.
Other apparatus is available which combines spark gap devices with non-linear resistors but they all have a leakage current or are unable to handle the power involved in protecting the secondary of a current transformer.
Protective apparatus for performing related functions is also known. For example, Canadian Pat. No. 887,797 - Thoren, issued Dec. 7, 1971 describes a device for limiting surges or overvoltages in AC power networks. A spark gap device or surge diverter is connected in series with the primary windings of a current transformer between the network and ground. The secondary winding of the current transformer is connected to operate a relay which closes another circuit connecting the network to ground. This circuit includes another spark gap device in parallel with an inductance. This arrangement is quite complex and not particularly suited for use in limiting voltages in the secondary of a current transformer.
Yet another protective apparatus is described in Canadian Pat. No. 158,414 - May, issued Oct. 20, 1914. This describes a vacuum lightning arrester with two electrodes defining a spark gap. A coiled strip of conductive metal is restrained in its coiled position by a spot of solder. If an arc current causes too high a temperature in the coil, the solder melts and the coil unwinds and shorts the electrodes. This lightning arrester provides a one-shot protection against unusual currents. It does, however, have a leakage current and it is of a complex design making it unsuitable for use in the secondary of a current transformer.